Based on the information present in the given passage, you need to write a correct answer to the given question. A correct answer is one that correctly and completely answers the question. Do not write answers that address the question partially or incorrectly. The correct answer must be contained in the given paragraph.

Paragraph- Sent 1: Heat supplies the energy that causes motion.
Sent 2: The deeper you go into toward the Earths core, the hotter it gets.
Sent 3: At the core it is really hot.
Sent 4: All that heat tries to rise toward the surface.
Sent 5: In the Mantle, the rock is partially melted and is able to move.
Sent 6: As it is heated, the material in the mantle moves toward the surface.
Sent 7: As the mantle material rises, it cools.
Sent 8: When it reaches the Earths crust, it is mostly stopped.
Sent 9: A little of the material can break through the surface, but not all.
Sent 10: Instead, it begins to move horizontally.
Sent 11: The mantle material moves horizontally away from a mid-ocean ridge crest.
Sent 12: Toward the surface, the mantle material starts to cool.
Sent 13: As it cools it sinks back down into the mantle.
Sent 14: These areas are where deep sea trench occur.
Sent 15: The material sinks back down to the core.
Sent 16: The system operates like a giant conveyor belt.
Sent 17: The motion due to heating and cooling is called convection. 
Question: What kind of feature occurs where cooled mantle material sinks back down into the mantle?.
Sea trench.